In the manufacturing of a product, the product is usually processed at many work stations or processing machines. The transporting or conveying of partially-finished products, or work-in-process (WIP) parts, is an important aspect in the total manufacturing process. The careful conveying of semiconductor wafers is especially important in the manufacturing of integrated circuit chips due to the delicate nature of the chips. Furthermore, in fabricating an IC product, a multiplicity of fabrication steps, i.e., as many as several hundred, is usually required to complete the fabrication process. A semiconductor wafer or IC chip must be transported between various process stations in order to facilitate various fabrication processes.
For instance, to complete the fabrication of an IC chip, various steps of deposition, cleaning, ion implantation, etching, and passivation must be carried out before an IC chip is packaged for shipment. Each of these fabrication steps must be performed in a different process machine, i.e., a chemical vapor deposition chamber, an ion implantation chamber, an etcher, etc. A partially processed semiconductor wafer must be conveyed between various work stations many times before the fabrication process is completed. The safe conveying and accurate tracking of such semiconductor wafers or work-in-process parts in a semiconductor fabrication facility is therefore an important aspect of the total fabrication process.
Conventionally, partially finished semiconductor wafers or WIP parts are conveyed in a fabrication plant by automatically-guided vehicles (AGVs) or overhead transport vehicles (OHTs) that travel on predetermined routes or tracks. For the conveying of semiconductor wafers, the wafers are normally loaded into cassettes or SMIF (standardized mechanical interface) pods and then picked up and placed in the automatic conveying vehicles. For identifying and locating the various semiconductor wafers or WIP parts being transported, the cassettes or pods are normally labeled with a tag positioned on the side of the cassette or pod. The tags can be read automatically by a tag reader that is mounted on the guard rails of the conveying vehicle. The AGVs and OHTs normally transport the pods from bay to bay along an interbay loop, and eventually deliver the pods to a robotic storage house, or “stocker”, which automatically delivers the pods to an intrabay loop.
In an automatic material handling system (AMHS), stockers are widely used in conjunction with automatically guided or overhead transport vehicles, either on the ground or suspended on tracks, for the storing and transporting of semiconductor wafers in SMIF pods or in wafer cassettes. For instance, as shown in FIG. 1 of the drawings, three possible configurations for utilizing a stocker are illustrated. In case A, a stocker 10 is utilized for storing WIP wafers in SMIF pods and transporting them first to tool A, then to tool B, and finally to tool C for three separate processing steps to be conducted on the wafers. After the processing in tool C is completed, the SMIF pod is returned to a stocker 10 for possible conveying to another stocker. The configuration shown in case A is theoretically workable but hardly ever possible in a fabrication environment, since the tools or processing equipment cannot always be arranged nearby to accommodate the processing of wafers in the stocker 10.
In the second configuration, case B shown in FIG. 1, a stocker 12 and a plurality of buffer stations A, B and C are used to accommodate three different processes to be conducted in tool A, tool B and tool C, respectively. As shown in FIG. 1, a SMIF pod may be first delivered to buffer station A from the stocker 12 and waits there for processing in tool A. Buffer stations B and C are similarly utilized in connection with tools B and C, respectively. The buffer stations A, B and C therefore become holding stations for conducting processes on the wafers. This configuration provides a workable solution to the fabrication process, but requires excessive floor space because of the additional buffer stations required. The configuration is therefore not feasible for use in a semiconductor fabrication facility.
In the third configuration, shown as case C in FIG. 1, a stocker 14 is provided for controlling the storage and conveying of WIP wafers to tools A, B and C. It is seen that after a SMIF pod is delivered to one of the three tools, the SMIF pod is always returned to to the stocker 14 before it is sent to the next processing tool. This is a viable process since only one stocker is required for handling three different processing tools and no buffer station is needed. The configuration shown in case C illustrates that the frequency of use of the stocker is extremely high since the stocker itself is used as a buffer station for all three tools. The accessing of the stocker 14 is therefore much more frequent than that required in the previous two configurations.
FIG. 2 illustrates a schematic of a typical automatic material handling system 20 that utilizes a central corridor 22, a plurality of bays 24 and a multiplicity of process machines 26. A multiplicity of stockers 30 are utilized for providing input/output to the bay 24, or to the processing machines 26 located on the bay 24. The central corridor 22 designed for bay layout is frequently used in an efficient automatic material handling system to perform lot transportation between bays. In this configuration, the stockers 30 of the automatic material handling system become the pathway for both input and output of the bay. Unfortunately, the stocker 30 frequently becomes a bottleneck for internal transportation. It has been observed that a major cause for the bottlenecking at the stockers 30 is the input/output ports of the stockers.
In modern semiconductor fabrication facilities, especially for the 200 mm or 300 mm FAB plants, automatic guided vehicles (AGV) and overhead transport vehicles (OHT) are extensively used to automate the wafer transport process as much as possible. The AGV and OHT utilize the input/output ports of a stocker to load or unload wafer lots, i.e., normally stored in wafer containers such as SMIF pods or FOUPs (front opening unified pods), for example. An overhead buffer (OHB) is typically provided near each process tool for the temporary storage of wafer containers prior to entry of each container into the process tool.
FIG. 3 is a perspective view of an overhead buffer (OHB) 32 including two vehicles 34, 36 that travel on a track 38. Both an input port 40 and an output port 42 are provided on the stocker 30. Each vehicle 36 stops at the input port 40 to place a wafer container 44 in the stocker 30, while wafers (not shown) in the wafer container 44 await processing at a processing tool in the vicinity of the stocker 30. An additional vehicle 36 either places an additional wafer container 44 in the input port 40 or retrieves a wafer container 44 from the output port 42 of the stocker 30, depending on the availability of the next processing tool (not shown) in the fabrication sequence for processing of wafers contained in the wafer container 44.
One limitation of the OHB 32 is that the OHB 32 is capable of accommodating only one vehicle 34 at a time. This causes considerable bottlenecking of multiple vehicles 34 at the input side or outlet side of the stocker 30. Therefore, a high-efficiency buffer stocker is needed for absorbing and facilitating the orderly and efficient flow of multiple transport vehicles which transport wafer containers containing wafers to a stocker or from a stocker to a process tool.